1. Field of the Invention
The present invention is related to an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film, typically a thin-film transistor) built on a substrate, and to an electronic device having the EL display device as a display portion.
2. Description of the Related Art
Techniques of forming a thin-film transistor (hereafter referred to as a TFT) on a substrate have greatly advanced in recent years, and development of applications to active matrix display devices is proceeding. In particular, a TFT using a polysilicon film has a higher electric field effect mobility than a TFT using a conventional amorphous silicon film, and therefore it is capable of high speed operation. It therefore becomes possible to perform control of a pixel, conventionally performed by a driver circuit external to the substrate, by a driver circuit formed on the same substrate as the pixel.
This type of active matrix display device is gaining attention for obtaining many advantages, such as lowered manufacturing cost, smaller display device, increased yield, and reduced throughput, by building various circuits and elements on the same substrate.
In addition, research on active matrix EL display devices having EL elements as self-luminescing elements has become spirited. The EL display device is also referred to as organic EL display (OELD) or an organic light emitting diode (OLED).
The EL display device is a self luminescing type which differs from a liquid crystal display device. The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes, and the EL layer is normally a lamination structure. A lamination structure of a hole transporting layer, a light emitting layer, and an electron transporting layer proposed by Tang, et al., of Eastman Kodak Co. can be given as a typical structure. This structure has extremely high light emitting efficiency, and nearly all EL display devices in which research and development is proceeding employ, this structure.
Furthermore, additional structures such as a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer formed in order on a pixel electrode; or a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer formed in order on a pixel electrode may also be used. An element such as a fluorescing pigment may also be doped into the EL layer.
A predetermined voltage is then applied to the EL layer having the above structure by a pair of electrodes, and recombination of a carrier thus occurs in the light emitting layer, and light is emitted.
The EL display device has roughly four colorizing display systems, that is, a system in which an EL element of white luminescence and color filters are combined, a system in which three kinds of EL elements corresponding to R (red), G (green) and B (blue) are formed, a system in which an EL element of blue or blue-green luminescence and a fluorescent material (fluorescent color converting layer: CCM) are combined, and a system in which a transparent electrode is used as a cathode (counter electrode) and EL elements corresponding to RGB are overlapped.
The color filter is a color filter for extracting light of red, green or blue. The color filter is formed at a position corresponding to a pixel, and the color of extracted light at each pixel can be changed by this. This is the same as a colorizing system of a liquid crystal display device using color filters in principal. Note that the position corresponding to the pixel indicates a position coincident with a pixel electrode.
However, the color filter is a filter which extracts the light of a specified wavelength to improve the color purity of the transmitted light. Thus, in the case where a light component of a wavelength to be extracted is small, there can occur such a disadvantage that the brightness of the light of the wavelength is extremely low or the color purity is poor.
In well-known organic EL materials, a red with high luminescent brightness has not been realized, and like an example shown in FIG. 10, the luminescent brightness of red is low as compared with the luminescent brightness of blue or green. In the case where an organic EL material having such luminescent characteristics is used for an EL display device, the luminescent brightness of red of an image to be displayed becomes poor.
Further, a method of using orange color light having a slightly lower wavelength than that of red color light has been conventionally performed because the brightness of red color light is low compared to the brightness of blue and green light. However, in this case the brightness of red color images which are displayed by the EL display device is low, and when display of the red color image is attempted, it is displayed as orange.